In myocardial cells from chick embryos, cultured with a method which facilitates cell differentiation and maturation, the ionic mechanisms responsible for heart-rate dependent changes of the transmembrane action potential will be studied by means of ion-sensitive micro-electrodes under varying concentrations of Na, K, Ca and substances which inhibit the flow of these ions (tetradotoxin, verapamil, tetraethyl ammonium). The T wave configuration on the chest surface of dogs will be compared with the distribution of myocardial repolarization in the ventricle by pulling fine dipole wire electrodes with one prominent and one recessed pole, combined with thermocouples, through the intact chest and various regions of the ventricles. In children 2 to l6 years old, the effect of deep inspiration on the T wave distribution over the thorax will be studied with especial reference to the "juvenile" T wave pattern. BIBLIOGRAPHIC REFRENCES: E. Lepeschkin. Physiological influences on transfer factors between myocardial currents and body surface potentials. In C. V. Nelson; "Theoretical Basis of Electrocardiology," Clarendon Press, London, l976. (In print). E. Lepeschkin: Role of Myocardial Temperature, Electrolyte and Stress Gradients on the Genesis of the Normal T Wave. In "Advances in Electrocardiography II". Ed. by R.D. Schlant and J.W. Hurst, Grune and Stratton, N.Y., 1976 (in print).